Secondary electron emitter and method of making it



May 23, 1939. c:.' F. VEENEMANS ET AL 9,77

SECONDARY ELECTRON EMITTER AND METHOD. OF MAKING IT Filed March -15,1958 INVENTORS CORNELIS FREDERIKVEENEMANS EDMUND HEINRICH LOPP HAJOBRUINING. B wdl/r ATTORNEY through a gas atmosphere having a sufilcientpres- .30 sure.

Patented May 23, 1939 PATENT oFi-"lcs SECONDARY ELECTRON EMITTER. ANDMETHOD or MAKING rr Cornelia Frederik .Veenemans, Edmund Heinrich Ltipp,and Hajo Bruining, Eindhoven, Netherlands, assignors to N. V. PhilipsGloeilampenfabrleken, Eindhoven, Netherlands Application March 15, 1938,Serial No. 195.942

In the Netherlands March 30, 1937 4 Claims. (CL 250-174) This inventionrelates to an electric discharge tube comprising a secondary electronemitting .electrode or emitter with at least a part of its surfaceconsisting of a substance readily emittin secondary electrons whenstruck by a stream of primary electrons t Various materials have beenused for secondary electron emission electrodes, such as the alkalimetals and their oxides, for example, caesium, l and the alkaline earthmetals such as barium and strontium and their oxides, and in additionmagnesium and aluminum and their oxides have been suggested. Generallythese electrodes or emit-- ters can be made by methods of manufacture vknown for primary thermionic cathodesffor instance, by applying thematerial containing the active metal to the secondary electron emissionelectrode by vaporization from a primary cathode. we have found thatexcellent secondary electron emittters may be made in accordance withour invention. According to the invention, in an electric discharge tubecomprising a secondary GQ litfsifil} electrode or emitter the activesubstance fionsisting of a compound of a metal 'selected When the metalitself isevaporated, we prefer to evaporate it through an atmosphere ofi a gas reacting with the metaland combining with the secondary emissionelectrode. For this purpose the gas pressure must have a value such that{35 the compound, and not merely a metal surface is produced. Invaporizing a metal compound either a reacting or inert gas atmospheremay be used. In vaporizing the compounds in a gas atmosphere, whetherreactive or not, thepressure 4c of the gas need not be as high as forthe vaporization of the metals themselves. We have found that in thismanner a much better secondary emission is obtained than when themetals, whether mixed with compounds or not, 45 are evaporated in a highvacuum, an advantage which may perhaps be due to the influence of thestructure of the coatings on the secondary electron emission propertiesof the secondary emission electrodes.

50 The method, according to the presentinvention, maybe carried out indiflerent ways. According to one way, a source of alkali metal, alkalineearth metal, or magnesium, either the metal or a readily decomposablecompound of the l 55. metal, is secured on another electrode in adischarge tube having a secondary emission electrode, the metal beingevaporated after a gas such as carbon dioxide, oxygen, and the like, forexample, has been introduced at a suitable pressure into the tube. In,this case there is formed on 6 the surface of the secondary emissionelectrode an oxide whose secondary emission is appreciably higher thanthat of the pure metal. According to another way, a carbon dioxideatmosphere is obtained by providing barium-strontium car- 10 bonate on aseparate heatable electrode, and ma nesium at another point of the tube,and by heating the carbonate shortly before vaporization of themagnesium to such a degree that carbon dioxide is set free and part, ofthe re- 1 suiting barium and strontium oxide also evaporates to thesecondary electron emitter, so that the secondary electron emissionelectrode is finally coated with a mixture of barium oxide, strontiumoxide, and magnesium oxide having a very good secondary electronemission and a ratio of secondary to primary electrons much greater thanunity at practicable voltages.

The invention will be more clearly understood by reference to theaccompanying drawing representing, by wayof example, a discharge tube inwhich the method of the invention may be used. This tube comprises .acathode 2, coated with barium and strontium oxides, a control grid 3,and a screen grid 4. The electrons issuing from the cathodeare led alongcurved paths, schematically represented by the arrows 5, to a secondaryemission electrode 6 and an anode I. The secondary electron emittingelectrode 6 is preferably a sheet of nickel having on its active surfacea thin layer or film of barium oxide, strontium oxide, and magnesiumoxide applied in accordance with our invention. In addition, the tubecomprises a curved accelerating electrode 8 and a shield 9 which serveto direct the electron discharge along curved paths and at the same timeprevent material evaporating from the primary cathode from reaching thesecondary emission electrode. The accelerating electrode 8 is sprayedwith a thin layer Ill of barium-strontium I 2 e I T 2,159}?! magnesium Il on the shield 9 is evaporated in the atmosphere of carbon dioxide andforms magnesium oxide on the electrode 6, In this case care has to betaken that the carbon dioxide is at 'such a pressure that no surface ofmetallic magnesium is formed. Usually the easily detectable pressuredeveloped in the tube by the decomposition of the carbonates issuilicient. As a result,

the surface of the nickel electrode is coated .with

10 a layer ofactive material in the form of an intimateadmixtureconsisting predominantly of oxides from the barium strontium carbonatecoating, and of the oxide or compound of the "vaporized metal, such asmagnesium. Usually metallic magnesium is not observable on the coating,although it is possible that minute amounts of metal are present in it;

We claim:

1. The method of forming in a tube a metal electrode having a surface ofhigh secondary electron emissivity which comprises evacuating the tube,heating an alkaline earth metal carbonate in the tube to a temperatureat which the carbonate decomposes with'evolutlonvof carbon di- 7 oxidegas and some alkaline earth metal oxide, and

evaporating a metal selected from the group consisting of the alkalineearth metals and mag-- nesium through the carbon dioxide atmosphere tothe surface of the metal electrode.

2. A secondary electron emitter comprising a sheet of nickel having asurface coated with a mixture consisting predominantly of an alkaline 5earth metal oxide and of magnesium oxide.

3. A secondary electron emitter comprising sheet of nickel having asurface coated with a mixture consisting predominantly of an alkalineearth metal oxide and not an oxide of a dlficrent o metal selected iromthe group comprising the alkali metals, the alkaline earth metals, and

magnesium. 4. The method of making a secondary electron emittingelectrode which comprises enclosing a 5 metal electrode in an evacuatedvessel, produc ing in'said vessel an atmosphere of carbon dioxide,depositing on said metal electrode in said atmosphere a thin him ofalkaline earth metal oxide, and vaporizing in said atmosphere a difmterent metal selected from the group of alkali metals, alkaline earthmetals, and magnesium to form a layer of oxide of said different metalon said fllm of alkaline earth metal oxides.

conmems mmanan: VEENEMANS.

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